Prior art methods for producing a black surface coating involve such procedures as depositing on the substrate a coating of black paint, a coating of black surface oxides or metallic compounds, a black coating of metal alloys, or a black coating of mixed metals.
A number of prior methods have been used to blacken areas to produce what is referred to as "optical black" surfaces. In the early work of Decker described in U.S. Pat. No. 3,867,207 he describes a process of blackening a component by using an electroless plating bath selected from the group consisting of nickel and cobalt and after subsequent rinsing immersing in an oxidizing acid bath of phosphoric, sulfuric and nitric acids and thereafter firing the component to form the blackened surface. More recently a process for producing black surface coatings has been described in U.S. Pat. No. 4,233,107 as well as U.S. Pat. No. 4,361,630 to Johnson, Sr., which involves preparing a substrate by cleaning and/or activating it and immersing the thus prepared substrate in an electroless plating bath containing nickel and hypophosphite ions in solution until an electroless nickel-phosphorus alloy coating has been deposited on the substrate. Thereafter, the substrate, coated with the electroless nickel-phosphorus alloy which has been washed and dried, is immersed in in an etchant bath consisting of an aqueous solution of nitric acid, wherein the nitric acid concentration ranges from a 1:5 a ratio with distilled or deionized water to concentrated until the coated surface of the substrate develops ultra-blackness.
While the process described in the aforementioned Johnson, Sr., patent provides a highly blackened surface on a substrate, it has been found that the emissivity capabilities thereof are limited such that its primary use is as a solar collector in the field of solar energy. In contradistinction, the present invention is not so limited in that substrates produced in accordance with the present invention have a highly blackened surface characterized by high infrared emissivities, as well as high absorptive capabilities, thus making them extremely suitable for use in such devices as infrared telescopes and sensors.